1. Field of the Invention
The present invention relates to a wireless channel setting method for a mobile communication system and a mobile communication control apparatus.
2. Related Background Art
A mobile communication system, called “FOMA” (product name) system, that employs CDMA (Code Division Multiple Access) is well known as an IMT-2000 wireless access technique. For this mobile communication system, the following method is employed to set up a wireless channel between a mobile station and a wireless base station.
Since the wireless capacity with the CDMA technology is limited by the level of the interference from other mobile stations located within the same cell, the minimum required power must be employed by all mobile stations in the same cell when communicating with each other in order to reduce interference. As a result with regard to the conventional mobile communication system, the wireless quality between a wireless base station and a mobile station must be measured all the time, and fast transmission power control must be executed in accordance with the measurement results. In this manner, optimization of the transmission power can be achieved, and an improved system capacity and a stable transmission quality can be obtained.
Generally, wireless channels used for a wireless communication system include a “common channel”, which is used by all mobile stations for transmission/reception when initiating communication, and an “individual channel”, which is allocated for each mobile station as paired uplink and downlink. Consequently, when initiating communication, a mobile station first accesses the common channel, through which control signals are exchanged to set up an individual channel. Thereafter, the wireless channel of the mobile station is shifted to the individual channel to perform voice or data communication.
A “control signal”, constituted by an indispensable group of signals transmitted to initiate communication between a mobile station and a network, includes data for identifying the type of mobile station and the requested service for verifying the mobile station, and for setting a confidentiality level. To accomplish all these matters, several signal exchanges are generally required.
Similarly, the FOMA system employs procedures for shifting from a common channel to an individual channel. Since the above described fast transmission power control can be applied only for the individual channel, which is a bi-directional channel, it is preferable that, while taking both the system capacity and the communication quality into account, the shift to the individual channel takes place as early as possible.
Therefore, conventionally, the channel switching process with a sequence in FIG. 4 is performed in the following manner. When a mobile station transmits a control signal over the common channel (sequence Q1), the wireless base station uses the common channel to designate an individual channel for control use (sequence Q2). Thereafter, the transmitting mobile station and the wireless base station switch the common channel to the individual channel for control use and exchange control signals (sequences Q3 and Q4). When the transmission/reception of the control signals has been completed, the wireless base station notifies the transmitting mobile station of an individual channel for communication use (sequence Q5), and in accordance with the communication type, such as voice communication or packet data communication, the two stations switch the current channel to the individual channel for communication use and again exchange wireless signals (sequences Q6 and Q7).
According to a wireless channel switching control method used in the conventional mobile communication system, the transmission/reception of signals over the common channel can be minimized, and individual channels can be employed for the exchange of almost all control signals. Therefore, greater effects can be realized by the fast transmission power control with regard to the individual channels.
However, according to a system such as the conventional mobile communication system, whereby an individual channel for control use and an individual channel for communication use are sequentially set from the common channel, it takes a long time for a user to connect and start transmission and to be capable of voice or data transmission, which has been problem. There are two reasons, the first of which is that, for a mobile station and a network, the layer 1 switching process, such as a change in the transmission rate, is accompanied by the switching of channels, and thus, a period of about 100 milliseconds, and up to seconds in a long case is required for the layer 1 switching process. The second reason is that the transmission rate for an individual channel for control use is lower than that for the common channel, so that, accordingly, an extended time period is required for the transmission of control signals. This second reason is due to the following. Since the common channel is shared by a plurality of mobile stations, the transmission rate provided for it is kept high to a degree. On the other hand, since the individual channel for control use is employed only by a pertinent mobile station, the transmission rate for the individual channel can not be thoroughly increased while taking the wireless and hardware resources into account if these are in proportion to the transmission rate.
As a countermeasure to solve these conventional shortcomings, there is a possible method that uses the common channel for exchanging all the control signals and that directly sets up the individual channel for communication use.
However, according to this countermeasure, regarding the state when the common channel is used, because of the system configuration, fast transmission power control is impossible, and depending on the conditions at the mobile station such as the current location thereof and the utilization thereof, the system capacity and the communication quality obtained may be unsatisfactory, compared with the state wherein an individual channel is used. Further, since the transmission rate for the common channel is higher and the peak transmission power is greater compared with those for an individual channel, the uplink and downlink paths through the common channel tend to act as interference sources for other mobile stations, especially when the pertinent mobile station is located near the edge of a cell. Furthermore, when the transmission power reaches the upper limit, a satisfactory communication quality is apt to not be obtained. And in addition, when multiple mobile stations are connected to the common channel, a delay in the uplink random access and a delay in the downlink transmission waiting may occur due to a contention control, and the connection time may be further extended.